
///
// optional - An implementation of std::optional with extensions
// Written in 2017 by Sy Brand (tartanllama@gmail.com, @TartanLlama)
//
// Documentation available at https://tl.tartanllama.xyz/
//
// To the extent possible under law, the author(s) have dedicated all
// copyright and related and neighboring rights to this software to the
// public domain worldwide. This software is distributed without any warranty.
//
// You should have received a copy of the CC0 Public Domain Dedication
// along with this software. If not, see
// <http://creativecommons.org/publicdomain/zero/1.0/>.
///

#ifndef TL_OPTIONAL_HPP
#define TL_OPTIONAL_HPP

#define TL_OPTIONAL_VERSION_MAJOR 1
#define TL_OPTIONAL_VERSION_MINOR 1
#define TL_OPTIONAL_VERSION_PATCH 0

#include <exception>
#include <functional>
#include <type_traits>
#include <utility>

#if (defined(_MSC_VER) && _MSC_VER == 1900)
#define TL_OPTIONAL_MSVC2015
#endif

#if (defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ <= 9 && !defined(__clang__))
#define TL_OPTIONAL_GCC49
#endif

#if (defined(__GNUC__) && __GNUC__ == 5 && __GNUC_MINOR__ <= 4 && !defined(__clang__))
#define TL_OPTIONAL_GCC54
#endif

#if (defined(__GNUC__) && __GNUC__ == 5 && __GNUC_MINOR__ <= 5 && !defined(__clang__))
#define TL_OPTIONAL_GCC55
#endif

#if (defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ <= 9 && !defined(__clang__))
// GCC < 5 doesn't support overloading on const&& for member functions
#define TL_OPTIONAL_NO_CONSTRR

// GCC < 5 doesn't support some standard C++11 type traits
#define TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) std::has_trivial_copy_constructor<T>::value
#define TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T)    std::has_trivial_copy_assign<T>::value

// This one will be different for GCC 5.7 if it's ever supported
#define TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T)       std::is_trivially_destructible<T>::value

// GCC 5 < v < 8 has a bug in is_trivially_copy_constructible which breaks
// std::vector for non-copyable types
#elif (defined(__GNUC__) && __GNUC__ < 8 && !defined(__clang__))
#ifndef TL_GCC_LESS_8_TRIVIALLY_COPY_CONSTRUCTIBLE_MUTEX
#define TL_GCC_LESS_8_TRIVIALLY_COPY_CONSTRUCTIBLE_MUTEX

namespace tl
{
  namespace detail
  {
    template <class T> struct is_trivially_copy_constructible : std::is_trivially_copy_constructible<T>
    {};
#ifdef _GLIBCXX_VECTOR
    template <class T, class A>
    struct is_trivially_copy_constructible<std::vector<T, A>> : std::is_trivially_copy_constructible<T>
    {};
#endif
  } // namespace detail
} // namespace tl
#endif

#define TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) tl::detail::is_trivially_copy_constructible<T>::value
#define TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T)    std::is_trivially_copy_assignable<T>::value
#define TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T)       std::is_trivially_destructible<T>::value
#else
#define TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) std::is_trivially_copy_constructible<T>::value
#define TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T)    std::is_trivially_copy_assignable<T>::value
#define TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T)       std::is_trivially_destructible<T>::value
#endif

#if __cplusplus > 201103L
#define TL_OPTIONAL_CXX14
#endif

// constexpr implies const in C++11, not C++14
#if (__cplusplus == 201103L || defined(TL_OPTIONAL_MSVC2015) || defined(TL_OPTIONAL_GCC49))
#define TL_OPTIONAL_11_CONSTEXPR
#else
#define TL_OPTIONAL_11_CONSTEXPR constexpr
#endif

namespace tl
{
#ifndef TL_MONOSTATE_INPLACE_MUTEX
#define TL_MONOSTATE_INPLACE_MUTEX

  /// Used to represent an optional with no data; essentially a bool
  class monostate
  {};

  ///  A tag type to tell optional to construct its value in-place
  struct in_place_t
  {
    explicit in_place_t() = default;
  };

  /// A tag to tell optional to construct its value in-place
  static constexpr in_place_t in_place{};
#endif

  template <class T> class optional;

  namespace detail
  {
#ifndef TL_TRAITS_MUTEX
#define TL_TRAITS_MUTEX
    // C++14-style aliases for brevity
    template <class T> using remove_const_t                 = typename std::remove_const<T>::type;
    template <class T> using remove_reference_t             = typename std::remove_reference<T>::type;
    template <class T> using decay_t                        = typename std::decay<T>::type;
    template <bool E, class T = void> using enable_if_t     = typename std::enable_if<E, T>::type;
    template <bool B, class T, class F> using conditional_t = typename std::conditional<B, T, F>::type;

    // std::conjunction from C++17
    template <class...> struct conjunction : std::true_type
    {};

    template <class B> struct conjunction<B> : B
    {};

    template <class B, class... Bs>
    struct conjunction<B, Bs...> : std::conditional<bool(B::value), conjunction<Bs...>, B>::type
    {};

#if defined(_LIBCPP_VERSION) && __cplusplus == 201103L
#define TL_TRAITS_LIBCXX_MEM_FN_WORKAROUND
#endif

// In C++11 mode, there's an issue in libc++'s std::mem_fn
// which results in a hard-error when using it in a noexcept expression
// in some cases. This is a check to workaround the common failing case.
#ifdef TL_TRAITS_LIBCXX_MEM_FN_WORKAROUND
    template <class T> struct is_pointer_to_non_const_member_func : std::false_type
    {};

    template <class T, class Ret, class... Args>
    struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...)> : std::true_type
    {};

    template <class T, class Ret, class... Args>
    struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...) &> : std::true_type
    {};

    template <class T, class Ret, class... Args>
    struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...) &&> : std::true_type
    {};

    template <class T, class Ret, class... Args>
    struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...) volatile> : std::true_type
    {};

    template <class T, class Ret, class... Args>
    struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...) volatile &> : std::true_type
    {};

    template <class T, class Ret, class... Args>
    struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...) volatile &&> : std::true_type
    {};

    template <class T> struct is_const_or_const_ref : std::false_type
    {};

    template <class T> struct is_const_or_const_ref<T const &> : std::true_type
    {};

    template <class T> struct is_const_or_const_ref<T const> : std::true_type
    {};
#endif

    // std::invoke from C++17
    // https://stackoverflow.com/questions/38288042/c11-14-invoke-workaround
    template <
      typename Fn,
      typename... Args,
#ifdef TL_TRAITS_LIBCXX_MEM_FN_WORKAROUND
      typename = enable_if_t<!(is_pointer_to_non_const_member_func<Fn>::value && is_const_or_const_ref<Args...>::value)>,
#endif
      typename = enable_if_t<std::is_member_pointer<decay_t<Fn>>::value>,
      int      = 0>
    constexpr auto invoke(Fn &&f, Args &&...args) noexcept(noexcept(std::mem_fn(f)(std::forward<Args>(args)...)))
      -> decltype(std::mem_fn(f)(std::forward<Args>(args)...))
    {
      return std::mem_fn(f)(std::forward<Args>(args)...);
    }

    template <typename Fn, typename... Args, typename = enable_if_t<!std::is_member_pointer<decay_t<Fn>>::value>>
    constexpr auto invoke(Fn &&f, Args &&...args) noexcept(noexcept(std::forward<Fn>(f)(std::forward<Args>(args)...)))
      -> decltype(std::forward<Fn>(f)(std::forward<Args>(args)...))
    {
      return std::forward<Fn>(f)(std::forward<Args>(args)...);
    }

    // std::invoke_result from C++17
    template <class F, class, class... Us> struct invoke_result_impl;

    template <class F, class... Us>
    struct invoke_result_impl<F, decltype(detail::invoke(std::declval<F>(), std::declval<Us>()...), void()), Us...>
    {
      using type = decltype(detail::invoke(std::declval<F>(), std::declval<Us>()...));
    };

    template <class F, class... Us> using invoke_result = invoke_result_impl<F, void, Us...>;

    template <class F, class... Us> using invoke_result_t = typename invoke_result<F, Us...>::type;

#if defined(_MSC_VER) && _MSC_VER <= 1900
    // TODO make a version which works with MSVC 2015
    template <class T, class U = T> struct is_swappable : std::true_type
    {};

    template <class T, class U = T> struct is_nothrow_swappable : std::true_type
    {};
#else
    // https://stackoverflow.com/questions/26744589/what-is-a-proper-way-to-implement-is-swappable-to-test-for-the-swappable-concept
    namespace swap_adl_tests
    {
      // if swap ADL finds this then it would call std::swap otherwise (same
      // signature)
      struct tag
      {};

      template <class T> tag                swap(T &, T &);
      template <class T, std::size_t N> tag swap(T (&a)[N], T (&b)[N]);

      // helper functions to test if an unqualified swap is possible, and if it
      // becomes std::swap
      template <class, class> std::false_type can_swap(...) noexcept(false);
      template <class T, class U, class = decltype(swap(std::declval<T &>(), std::declval<U &>()))>
      std::true_type can_swap(int) noexcept(noexcept(swap(std::declval<T &>(), std::declval<U &>())));

      template <class, class> std::false_type uses_std(...);
      template <class T, class U>
      std::is_same<decltype(swap(std::declval<T &>(), std::declval<U &>())), tag> uses_std(int);

      template <class T>
      struct is_std_swap_noexcept
        : std::integral_constant<
            bool,
            std::is_nothrow_move_constructible<T>::value && std::is_nothrow_move_assignable<T>::value>
      {};

      template <class T, std::size_t N> struct is_std_swap_noexcept<T[N]> : is_std_swap_noexcept<T>
      {};

      template <class T, class U>
      struct is_adl_swap_noexcept : std::integral_constant<bool, noexcept(can_swap<T, U>(0))>
      {};
    } // namespace swap_adl_tests

    template <class T, class U = T>
    struct is_swappable
      : std::integral_constant<
          bool,
          decltype(detail::swap_adl_tests::can_swap<T, U>(0))::value
            && (!decltype(detail::swap_adl_tests::uses_std<T, U>(0))::value || (std::is_move_assignable<T>::value && std::is_move_constructible<T>::value))>
    {};

    template <class T, std::size_t N>
    struct is_swappable<T[N], T[N]>
      : std::integral_constant<
          bool,
          decltype(detail::swap_adl_tests::can_swap<T[N], T[N]>(0))::value
            && (!decltype(detail::swap_adl_tests::uses_std<T[N], T[N]>(0))::value || is_swappable<T, T>::value)>
    {};

    template <class T, class U = T>
    struct is_nothrow_swappable
      : std::integral_constant<
          bool,
          is_swappable<T, U>::value
            && ((decltype(detail::swap_adl_tests::uses_std<T, U>(0))::value && detail::swap_adl_tests::is_std_swap_noexcept<T>::value) || (!decltype(detail::swap_adl_tests::uses_std<T, U>(0))::value && detail::swap_adl_tests::is_adl_swap_noexcept<T, U>::value))>
    {};
#endif
#endif

    // std::void_t from C++17
    template <class...> struct voider
    {
      using type = void;
    };
    template <class... Ts> using void_t = typename voider<Ts...>::type;

    // Trait for checking if a type is a tl::optional
    template <class T> struct is_optional_impl : std::false_type
    {};

    template <class T> struct is_optional_impl<optional<T>> : std::true_type
    {};
    template <class T> using is_optional = is_optional_impl<decay_t<T>>;

    // Change void to tl::monostate
    template <class U> using fixup_void = conditional_t<std::is_void<U>::value, monostate, U>;

    template <class F, class U, class = invoke_result_t<F, U>>
    using get_map_return = optional<fixup_void<invoke_result_t<F, U>>>;

    // Check if invoking F for some Us returns void
    template <class F, class = void, class... U> struct returns_void_impl;

    template <class F, class... U>
    struct returns_void_impl<F, void_t<invoke_result_t<F, U...>>, U...> : std::is_void<invoke_result_t<F, U...>>
    {};
    template <class F, class... U> using returns_void = returns_void_impl<F, void, U...>;

    template <class T, class... U> using enable_if_ret_void = enable_if_t<returns_void<T &&, U...>::value>;

    template <class T, class... U> using disable_if_ret_void = enable_if_t<!returns_void<T &&, U...>::value>;

    template <class T, class U>
    using enable_forward_value = detail::enable_if_t<
      std::is_constructible<T, U &&>::value && !std::is_same<detail::decay_t<U>, in_place_t>::value
      && !std::is_same<optional<T>, detail::decay_t<U>>::value>;

    template <class T, class U, class Other>
    using enable_from_other = detail::enable_if_t<
      std::is_constructible<T, Other>::value && !std::is_constructible<T, optional<U> &>::value
      && !std::is_constructible<T, optional<U> &&>::value && !std::is_constructible<T, const optional<U> &>::value
      && !std::is_constructible<T, const optional<U> &&>::value && !std::is_convertible<optional<U> &, T>::value
      && !std::is_convertible<optional<U> &&, T>::value && !std::is_convertible<const optional<U> &, T>::value
      && !std::is_convertible<const optional<U> &&, T>::value>;

    template <class T, class U>
    using enable_assign_forward = detail::enable_if_t<
      !std::is_same<optional<T>, detail::decay_t<U>>::value
      && !detail::conjunction<std::is_scalar<T>, std::is_same<T, detail::decay_t<U>>>::value
      && std::is_constructible<T, U>::value && std::is_assignable<T &, U>::value>;

    template <class T, class U, class Other>
    using enable_assign_from_other = detail::enable_if_t<
      std::is_constructible<T, Other>::value && std::is_assignable<T &, Other>::value
      && !std::is_constructible<T, optional<U> &>::value && !std::is_constructible<T, optional<U> &&>::value
      && !std::is_constructible<T, const optional<U> &>::value && !std::is_constructible<T, const optional<U> &&>::value
      && !std::is_convertible<optional<U> &, T>::value && !std::is_convertible<optional<U> &&, T>::value
      && !std::is_convertible<const optional<U> &, T>::value && !std::is_convertible<const optional<U> &&, T>::value
      && !std::is_assignable<T &, optional<U> &>::value && !std::is_assignable<T &, optional<U> &&>::value
      && !std::is_assignable<T &, const optional<U> &>::value && !std::is_assignable<T &, const optional<U> &&>::value>;

    // The storage base manages the actual storage, and correctly propagates
    // trivial destruction from T. This case is for when T is not trivially
    // destructible.
    template <class T, bool = ::std::is_trivially_destructible<T>::value> struct optional_storage_base
    {
      TL_OPTIONAL_11_CONSTEXPR optional_storage_base() noexcept : m_dummy(), m_has_value(false) {}

      template <class... U>
      TL_OPTIONAL_11_CONSTEXPR optional_storage_base(in_place_t, U &&...u)
        : m_value(std::forward<U>(u)...), m_has_value(true)
      {}

      ~optional_storage_base()
      {
        if (m_has_value)
          {
            m_value.~T();
            m_has_value = false;
          }
      }

      struct dummy
      {};

      union
      {
        dummy m_dummy;
        T     m_value;
      };

      bool m_has_value;
    };

    // This case is for when T is trivially destructible.
    template <class T> struct optional_storage_base<T, true>
    {
      TL_OPTIONAL_11_CONSTEXPR optional_storage_base() noexcept : m_dummy(), m_has_value(false) {}

      template <class... U>
      TL_OPTIONAL_11_CONSTEXPR optional_storage_base(in_place_t, U &&...u)
        : m_value(std::forward<U>(u)...), m_has_value(true)
      {}

      // No destructor, so this class is trivially destructible

      struct dummy
      {};

      union
      {
        dummy m_dummy;
        T     m_value;
      };

      bool m_has_value = false;
    };

    // This base class provides some handy member functions which can be used in
    // further derived classes
    template <class T> struct optional_operations_base : optional_storage_base<T>
    {
      using optional_storage_base<T>::optional_storage_base;

      void hard_reset() noexcept
      {
        get().~T();
        this->m_has_value = false;
      }

      template <class... Args> void construct(Args &&...args)
      {
        new (std::addressof(this->m_value)) T(std::forward<Args>(args)...);
        this->m_has_value = true;
      }

      template <class Opt> void assign(Opt &&rhs)
      {
        if (this->has_value())
          {
            if (rhs.has_value())
              {
                this->m_value = std::forward<Opt>(rhs).get();
              }
            else
              {
                this->m_value.~T();
                this->m_has_value = false;
              }
          }

        else if (rhs.has_value())
          {
            construct(std::forward<Opt>(rhs).get());
          }
      }

      bool has_value() const { return this->m_has_value; }

      TL_OPTIONAL_11_CONSTEXPR T &get() & { return this->m_value; }

      TL_OPTIONAL_11_CONSTEXPR const T &get() const & { return this->m_value; }

      TL_OPTIONAL_11_CONSTEXPR T &&get() && { return std::move(this->m_value); }
#ifndef TL_OPTIONAL_NO_CONSTRR
      constexpr const T &&get() const && { return std::move(this->m_value); }
#endif
    };

    // This class manages conditionally having a trivial copy constructor
    // This specialization is for when T is trivially copy constructible
    template <class T, bool = TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)>
    struct optional_copy_base : optional_operations_base<T>
    {
      using optional_operations_base<T>::optional_operations_base;
    };

    // This specialization is for when T is not trivially copy constructible
    template <class T> struct optional_copy_base<T, false> : optional_operations_base<T>
    {
      using optional_operations_base<T>::optional_operations_base;

      optional_copy_base() = default;

      optional_copy_base(const optional_copy_base &rhs) : optional_operations_base<T>()
      {
        if (rhs.has_value())
          this->construct(rhs.get());
        else
          this->m_has_value = false;
      }

      optional_copy_base(optional_copy_base &&rhs)                 = default;
      optional_copy_base &operator=(const optional_copy_base &rhs) = default;
      optional_copy_base &operator=(optional_copy_base &&rhs)      = default;
    };

// This class manages conditionally having a trivial move constructor
// Unfortunately there's no way to achieve this in GCC < 5 AFAIK, since it
// doesn't implement an analogue to std::is_trivially_move_constructible. We
// have to make do with a non-trivial move constructor even if T is trivially
// move constructible
#ifndef TL_OPTIONAL_GCC49
    template <class T, bool = std::is_trivially_move_constructible<T>::value>
    struct optional_move_base : optional_copy_base<T>
    {
      using optional_copy_base<T>::optional_copy_base;
    };
#else
    template <class T, bool = false> struct optional_move_base;
#endif
    template <class T> struct optional_move_base<T, false> : optional_copy_base<T>
    {
      using optional_copy_base<T>::optional_copy_base;

      optional_move_base()                              = default;
      optional_move_base(const optional_move_base &rhs) = default;

      optional_move_base(optional_move_base &&rhs) noexcept(std::is_nothrow_move_constructible<T>::value)
      {
        if (rhs.has_value())
          this->construct(std::move(rhs.get()));
        else
          this->m_has_value = false;
      }

      optional_move_base &operator=(const optional_move_base &rhs) = default;
      optional_move_base &operator=(optional_move_base &&rhs)      = default;
    };

    // This class manages conditionally having a trivial copy assignment
    // operator
    template <
      class T,
      bool = TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T) && TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)
             && TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T)>
    struct optional_copy_assign_base : optional_move_base<T>
    {
      using optional_move_base<T>::optional_move_base;
    };

    template <class T> struct optional_copy_assign_base<T, false> : optional_move_base<T>
    {
      using optional_move_base<T>::optional_move_base;

      optional_copy_assign_base()                                     = default;
      optional_copy_assign_base(const optional_copy_assign_base &rhs) = default;

      optional_copy_assign_base(optional_copy_assign_base &&rhs) = default;

      optional_copy_assign_base &operator=(const optional_copy_assign_base &rhs)
      {
        this->assign(rhs);
        return *this;
      }

      optional_copy_assign_base &operator=(optional_copy_assign_base &&rhs) = default;
    };

// This class manages conditionally having a trivial move assignment operator
// Unfortunately there's no way to achieve this in GCC < 5 AFAIK, since it
// doesn't implement an analogue to std::is_trivially_move_assignable. We have
// to make do with a non-trivial move assignment operator even if T is trivially
// move assignable
#ifndef TL_OPTIONAL_GCC49
    template <
      class T,
      bool = std::is_trivially_destructible<T>::value && std::is_trivially_move_constructible<T>::value
             && std::is_trivially_move_assignable<T>::value>
    struct optional_move_assign_base : optional_copy_assign_base<T>
    {
      using optional_copy_assign_base<T>::optional_copy_assign_base;
    };
#else
    template <class T, bool = false> struct optional_move_assign_base;
#endif

    template <class T> struct optional_move_assign_base<T, false> : optional_copy_assign_base<T>
    {
      using optional_copy_assign_base<T>::optional_copy_assign_base;

      optional_move_assign_base()                                     = default;
      optional_move_assign_base(const optional_move_assign_base &rhs) = default;

      optional_move_assign_base(optional_move_assign_base &&rhs) = default;

      optional_move_assign_base &operator=(const optional_move_assign_base &rhs) = default;

      optional_move_assign_base &operator=(optional_move_assign_base &&rhs) noexcept(
        std::is_nothrow_move_constructible<T>::value && std::is_nothrow_move_assignable<T>::value)
      {
        this->assign(std::move(rhs));
        return *this;
      }
    };

    // optional_delete_ctor_base will conditionally delete copy and move
    // constructors depending on whether T is copy/move constructible
    template <
      class T,
      bool EnableCopy = std::is_copy_constructible<T>::value,
      bool EnableMove = std::is_move_constructible<T>::value>
    struct optional_delete_ctor_base
    {
      optional_delete_ctor_base()                                                 = default;
      optional_delete_ctor_base(const optional_delete_ctor_base &)                = default;
      optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept            = default;
      optional_delete_ctor_base &operator=(const optional_delete_ctor_base &)     = default;
      optional_delete_ctor_base &operator=(optional_delete_ctor_base &&) noexcept = default;
    };

    template <class T> struct optional_delete_ctor_base<T, true, false>
    {
      optional_delete_ctor_base()                                                 = default;
      optional_delete_ctor_base(const optional_delete_ctor_base &)                = default;
      optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept            = delete;
      optional_delete_ctor_base &operator=(const optional_delete_ctor_base &)     = default;
      optional_delete_ctor_base &operator=(optional_delete_ctor_base &&) noexcept = default;
    };

    template <class T> struct optional_delete_ctor_base<T, false, true>
    {
      optional_delete_ctor_base()                                                 = default;
      optional_delete_ctor_base(const optional_delete_ctor_base &)                = delete;
      optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept            = default;
      optional_delete_ctor_base &operator=(const optional_delete_ctor_base &)     = default;
      optional_delete_ctor_base &operator=(optional_delete_ctor_base &&) noexcept = default;
    };

    template <class T> struct optional_delete_ctor_base<T, false, false>
    {
      optional_delete_ctor_base()                                                 = default;
      optional_delete_ctor_base(const optional_delete_ctor_base &)                = delete;
      optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept            = delete;
      optional_delete_ctor_base &operator=(const optional_delete_ctor_base &)     = default;
      optional_delete_ctor_base &operator=(optional_delete_ctor_base &&) noexcept = default;
    };

    // optional_delete_assign_base will conditionally delete copy and move
    // constructors depending on whether T is copy/move constructible +
    // assignable
    template <
      class T,
      bool EnableCopy = (std::is_copy_constructible<T>::value && std::is_copy_assignable<T>::value),
      bool EnableMove = (std::is_move_constructible<T>::value && std::is_move_assignable<T>::value)>
    struct optional_delete_assign_base
    {
      optional_delete_assign_base()                                                   = default;
      optional_delete_assign_base(const optional_delete_assign_base &)                = default;
      optional_delete_assign_base(optional_delete_assign_base &&) noexcept            = default;
      optional_delete_assign_base &operator=(const optional_delete_assign_base &)     = default;
      optional_delete_assign_base &operator=(optional_delete_assign_base &&) noexcept = default;
    };

    template <class T> struct optional_delete_assign_base<T, true, false>
    {
      optional_delete_assign_base()                                                   = default;
      optional_delete_assign_base(const optional_delete_assign_base &)                = default;
      optional_delete_assign_base(optional_delete_assign_base &&) noexcept            = default;
      optional_delete_assign_base &operator=(const optional_delete_assign_base &)     = default;
      optional_delete_assign_base &operator=(optional_delete_assign_base &&) noexcept = delete;
    };

    template <class T> struct optional_delete_assign_base<T, false, true>
    {
      optional_delete_assign_base()                                                   = default;
      optional_delete_assign_base(const optional_delete_assign_base &)                = default;
      optional_delete_assign_base(optional_delete_assign_base &&) noexcept            = default;
      optional_delete_assign_base &operator=(const optional_delete_assign_base &)     = delete;
      optional_delete_assign_base &operator=(optional_delete_assign_base &&) noexcept = default;
    };

    template <class T> struct optional_delete_assign_base<T, false, false>
    {
      optional_delete_assign_base()                                                   = default;
      optional_delete_assign_base(const optional_delete_assign_base &)                = default;
      optional_delete_assign_base(optional_delete_assign_base &&) noexcept            = default;
      optional_delete_assign_base &operator=(const optional_delete_assign_base &)     = delete;
      optional_delete_assign_base &operator=(optional_delete_assign_base &&) noexcept = delete;
    };

  } // namespace detail

  /// A tag type to represent an empty optional
  struct nullopt_t
  {
    struct do_not_use
    {};

    constexpr explicit nullopt_t(do_not_use, do_not_use) noexcept {}
  };

  /// Represents an empty optional
  static constexpr nullopt_t nullopt{nullopt_t::do_not_use{}, nullopt_t::do_not_use{}};

  class bad_optional_access : public std::exception
  {
  public:
    bad_optional_access() = default;

    const char *what() const noexcept { return "Optional has no value"; }
  };

  /// An optional object is an object that contains the storage for another
  /// object and manages the lifetime of this contained object, if any. The
  /// contained object may be initialized after the optional object has been
  /// initialized, and may be destroyed before the optional object has been
  /// destroyed. The initialization state of the contained object is tracked by
  /// the optional object.
  template <class T>
  class optional : private detail::optional_move_assign_base<T>,
                   private detail::optional_delete_ctor_base<T>,
                   private detail::optional_delete_assign_base<T>
  {
    using base = detail::optional_move_assign_base<T>;

    static_assert(!std::is_same<T, in_place_t>::value, "instantiation of optional with in_place_t is ill-formed");
    static_assert(
      !std::is_same<detail::decay_t<T>, nullopt_t>::value, "instantiation of optional with nullopt_t is ill-formed");

  public:
// The different versions for C++14 and 11 are needed because deduced return
// types are not SFINAE-safe. This provides better support for things like
// generic lambdas. C.f.
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0826r0.html
#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54)                           \
  && !defined(TL_OPTIONAL_GCC55)
    /// Carries out some operation which returns an optional on the stored
    /// object if there is one.
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F &&f) &
    {
      using result = detail::invoke_result_t<F, T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }

    template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F &&f) &&
    {
      using result = detail::invoke_result_t<F, T &&>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : result(nullopt);
    }

    template <class F> constexpr auto and_then(F &&f) const &
    {
      using result = detail::invoke_result_t<F, const T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F> constexpr auto and_then(F &&f) const &&
    {
      using result = detail::invoke_result_t<F, const T &&>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : result(nullopt);
    }
#endif
#else
    /// Carries out some operation which returns an optional on the stored
    /// object if there is one.
    template <class F> TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T &> and_then(F &&f) &
    {
      using result = detail::invoke_result_t<F, T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }

    template <class F> TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T &&> and_then(F &&f) &&
    {
      using result = detail::invoke_result_t<F, T &&>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : result(nullopt);
    }

    template <class F> constexpr detail::invoke_result_t<F, const T &> and_then(F &&f) const &
    {
      using result = detail::invoke_result_t<F, const T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F> constexpr detail::invoke_result_t<F, const T &&> and_then(F &&f) const &&
    {
      using result = detail::invoke_result_t<F, const T &&>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : result(nullopt);
    }
#endif
#endif

#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54)                           \
  && !defined(TL_OPTIONAL_GCC55)
    /// Carries out some operation on the stored object if there is one.
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F &&f) &
    {
      return optional_map_impl(*this, std::forward<F>(f));
    }

    template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F &&f) &&
    {
      return optional_map_impl(std::move(*this), std::forward<F>(f));
    }

    template <class F> constexpr auto map(F &&f) const & { return optional_map_impl(*this, std::forward<F>(f)); }

    template <class F> constexpr auto map(F &&f) const &&
    {
      return optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#else
    /// Carries out some operation on the stored object if there is one.
    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional &>(), std::declval<F &&>())) map(F &&f) &
    {
      return optional_map_impl(*this, std::forward<F>(f));
    }

    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional &&>(), std::declval<F &&>())) map(F &&f) &&
    {
      return optional_map_impl(std::move(*this), std::forward<F>(f));
    }

    template <class F>
    constexpr decltype(optional_map_impl(std::declval<const optional &>(), std::declval<F &&>())) map(F &&f) const &
    {
      return optional_map_impl(*this, std::forward<F>(f));
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F>
    constexpr decltype(optional_map_impl(std::declval<const optional &&>(), std::declval<F &&>())) map(F &&f) const &&
    {
      return optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#endif
#endif

#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54)                           \
  && !defined(TL_OPTIONAL_GCC55)
    /// Carries out some operation on the stored object if there is one.
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F &&f) &
    {
      return optional_map_impl(*this, std::forward<F>(f));
    }

    template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F &&f) &&
    {
      return optional_map_impl(std::move(*this), std::forward<F>(f));
    }

    template <class F> constexpr auto transform(F &&f) const & { return optional_map_impl(*this, std::forward<F>(f)); }

    template <class F> constexpr auto transform(F &&f) const &&
    {
      return optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#else
    /// Carries out some operation on the stored object if there is one.
    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional &>(), std::declval<F &&>()))
      transform(F &&f) &
    {
      return optional_map_impl(*this, std::forward<F>(f));
    }

    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional &&>(), std::declval<F &&>()))
      transform(F &&f) &&
    {
      return optional_map_impl(std::move(*this), std::forward<F>(f));
    }

    template <class F>
    constexpr decltype(optional_map_impl(std::declval<const optional &>(), std::declval<F &&>()))
      transform(F &&f) const &
    {
      return optional_map_impl(*this, std::forward<F>(f));
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F>
    constexpr decltype(optional_map_impl(std::declval<const optional &&>(), std::declval<F &&>()))
      transform(F &&f) const &&
    {
      return optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#endif
#endif

    /// Calls `f` if the optional is empty
    template <class F, detail::enable_if_ret_void<F> * = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) &
    {
      if (has_value())
        return *this;

      std::forward<F>(f)();
      return nullopt;
    }

    template <class F, detail::disable_if_ret_void<F> * = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) &
    {
      return has_value() ? *this : std::forward<F>(f)();
    }

    template <class F, detail::enable_if_ret_void<F> * = nullptr> optional<T> or_else(F &&f) &&
    {
      if (has_value())
        return std::move(*this);

      std::forward<F>(f)();
      return nullopt;
    }

    template <class F, detail::disable_if_ret_void<F> * = nullptr>
    optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) &&
    {
      return has_value() ? std::move(*this) : std::forward<F>(f)();
    }

    template <class F, detail::enable_if_ret_void<F> * = nullptr> optional<T> or_else(F &&f) const &
    {
      if (has_value())
        return *this;

      std::forward<F>(f)();
      return nullopt;
    }

    template <class F, detail::disable_if_ret_void<F> * = nullptr>
    optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) const &
    {
      return has_value() ? *this : std::forward<F>(f)();
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, detail::enable_if_ret_void<F> * = nullptr> optional<T> or_else(F &&f) const &&
    {
      if (has_value())
        return std::move(*this);

      std::forward<F>(f)();
      return nullopt;
    }

    template <class F, detail::disable_if_ret_void<F> * = nullptr> optional<T> or_else(F &&f) const &&
    {
      return has_value() ? std::move(*this) : std::forward<F>(f)();
    }
#endif

    /// Maps the stored value with `f` if there is one, otherwise returns `u`.
    template <class F, class U> U map_or(F &&f, U &&u) &
    {
      return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u);
    }

    template <class F, class U> U map_or(F &&f, U &&u) &&
    {
      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u);
    }

    template <class F, class U> U map_or(F &&f, U &&u) const &
    {
      return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u);
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, class U> U map_or(F &&f, U &&u) const &&
    {
      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u);
    }
#endif

    /// Maps the stored value with `f` if there is one, otherwise calls
    /// `u` and returns the result.
    template <class F, class U> detail::invoke_result_t<U> map_or_else(F &&f, U &&u) &
    {
      return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u)();
    }

    template <class F, class U> detail::invoke_result_t<U> map_or_else(F &&f, U &&u) &&
    {
      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u)();
    }

    template <class F, class U> detail::invoke_result_t<U> map_or_else(F &&f, U &&u) const &
    {
      return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u)();
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, class U> detail::invoke_result_t<U> map_or_else(F &&f, U &&u) const &&
    {
      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u)();
    }
#endif

    /// Returns `u` if `*this` has a value, otherwise an empty optional.
    template <class U> constexpr optional<typename std::decay<U>::type> conjunction(U &&u) const
    {
      using result = optional<detail::decay_t<U>>;
      return has_value() ? result{u} : result{nullopt};
    }

    /// Returns `rhs` if `*this` is empty, otherwise the current value.
    TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional &rhs) & { return has_value() ? *this : rhs; }

    constexpr optional disjunction(const optional &rhs) const & { return has_value() ? *this : rhs; }

    TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional &rhs) &&
    {
      return has_value() ? std::move(*this) : rhs;
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    constexpr optional disjunction(const optional &rhs) const && { return has_value() ? std::move(*this) : rhs; }
#endif

    TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional &&rhs) & { return has_value() ? *this : std::move(rhs); }

    constexpr optional disjunction(optional &&rhs) const & { return has_value() ? *this : std::move(rhs); }

    TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional &&rhs) &&
    {
      return has_value() ? std::move(*this) : std::move(rhs);
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    constexpr optional disjunction(optional &&rhs) const && { return has_value() ? std::move(*this) : std::move(rhs); }
#endif

    /// Takes the value out of the optional, leaving it empty
    optional take()
    {
      optional ret = std::move(*this);
      reset();
      return ret;
    }

    using value_type = T;

    /// Constructs an optional that does not contain a value.
    constexpr optional() noexcept = default;

    constexpr optional(nullopt_t) noexcept {}

    /// Copy constructor
    ///
    /// If `rhs` contains a value, the stored value is direct-initialized with
    /// it. Otherwise, the constructed optional is empty.
    TL_OPTIONAL_11_CONSTEXPR optional(const optional &rhs) = default;

    /// Move constructor
    ///
    /// If `rhs` contains a value, the stored value is direct-initialized with
    /// it. Otherwise, the constructed optional is empty.
    TL_OPTIONAL_11_CONSTEXPR optional(optional &&rhs) = default;

    /// Constructs the stored value in-place using the given arguments.
    template <class... Args>
    constexpr explicit optional(
      detail::enable_if_t<std::is_constructible<T, Args...>::value, in_place_t>, Args &&...args)
      : base(in_place, std::forward<Args>(args)...)
    {}

    template <class U, class... Args>
    TL_OPTIONAL_11_CONSTEXPR explicit optional(
      detail::enable_if_t<std::is_constructible<T, std::initializer_list<U> &, Args &&...>::value, in_place_t>,
      std::initializer_list<U> il,
      Args &&...args)
    {
      this->construct(il, std::forward<Args>(args)...);
    }

    /// Constructs the stored value with `u`.
    template <
      class U                                                    = T,
      detail::enable_if_t<std::is_convertible<U &&, T>::value> * = nullptr,
      detail::enable_forward_value<T, U>                       * = nullptr>
    constexpr optional(U &&u) : base(in_place, std::forward<U>(u))
    {}

    template <
      class U                                                     = T,
      detail::enable_if_t<!std::is_convertible<U &&, T>::value> * = nullptr,
      detail::enable_forward_value<T, U>                        * = nullptr>
    constexpr explicit optional(U &&u) : base(in_place, std::forward<U>(u))
    {}

    /// Converting copy constructor.
    template <
      class U,
      detail::enable_from_other<T, U, const U &>                    * = nullptr,
      detail::enable_if_t<std::is_convertible<const U &, T>::value> * = nullptr>
    optional(const optional<U> &rhs)
    {
      if (rhs.has_value())
        this->construct(*rhs);
    }

    template <
      class U,
      detail::enable_from_other<T, U, const U &>                     * = nullptr,
      detail::enable_if_t<!std::is_convertible<const U &, T>::value> * = nullptr>
    explicit optional(const optional<U> &rhs)
    {
      if (rhs.has_value())
        this->construct(*rhs);
    }

    /// Converting move constructor.
    template <
      class U,
      detail::enable_from_other<T, U, U &&>                    * = nullptr,
      detail::enable_if_t<std::is_convertible<U &&, T>::value> * = nullptr>
    optional(optional<U> &&rhs)
    {
      if (rhs.has_value())
        this->construct(std::move(*rhs));
    }

    template <
      class U,
      detail::enable_from_other<T, U, U &&>                     * = nullptr,
      detail::enable_if_t<!std::is_convertible<U &&, T>::value> * = nullptr>
    explicit optional(optional<U> &&rhs)
    {
      if (rhs.has_value())
        this->construct(std::move(*rhs));
    }

    /// Destroys the stored value if there is one.
    ~optional() = default;

    /// Assignment to empty.
    ///
    /// Destroys the current value if there is one.
    optional &operator=(nullopt_t) noexcept
    {
      if (has_value())
        {
          this->m_value.~T();
          this->m_has_value = false;
        }

      return *this;
    }

    /// Copy assignment.
    ///
    /// Copies the value from `rhs` if there is one. Otherwise resets the stored
    /// value in `*this`.
    optional &operator=(const optional &rhs) = default;

    /// Move assignment.
    ///
    /// Moves the value from `rhs` if there is one. Otherwise resets the stored
    /// value in `*this`.
    optional &operator=(optional &&rhs) = default;

    /// Assigns the stored value from `u`, destroying the old value if there was
    /// one.
    template <class U = T, detail::enable_assign_forward<T, U> * = nullptr> optional &operator=(U &&u)
    {
      if (has_value())
        this->m_value = std::forward<U>(u);
      else
        this->construct(std::forward<U>(u));

      return *this;
    }

    /// Converting copy assignment operator.
    ///
    /// Copies the value from `rhs` if there is one. Otherwise resets the stored
    /// value in `*this`.
    template <class U, detail::enable_assign_from_other<T, U, const U &> * = nullptr>
    optional &operator=(const optional<U> &rhs)
    {
      if (has_value())
        if (rhs.has_value())
          this->m_value = *rhs;
        else
          this->hard_reset();

      else if (rhs.has_value())
        this->construct(*rhs);

      return *this;
    }

    // TODO check exception guarantee
    /// Converting move assignment operator.
    ///
    /// Moves the value from `rhs` if there is one. Otherwise resets the stored
    /// value in `*this`.
    template <class U, detail::enable_assign_from_other<T, U, U> * = nullptr> optional &operator=(optional<U> &&rhs)
    {
      if (has_value())
        if (rhs.has_value())
          this->m_value = std::move(*rhs);
        else
          this->hard_reset();

      else if (rhs.has_value())
        this->construct(std::move(*rhs));

      return *this;
    }

    /// Constructs the value in-place, destroying the current one if there is
    /// one.
    template <class... Args> T &emplace(Args &&...args)
    {
      static_assert(std::is_constructible<T, Args &&...>::value, "T must be constructible with Args");

      *this = nullopt;
      this->construct(std::forward<Args>(args)...);
      return value();
    }

    template <class U, class... Args>
    detail::enable_if_t<std::is_constructible<T, std::initializer_list<U> &, Args &&...>::value, T &>
      emplace(std::initializer_list<U> il, Args &&...args)
    {
      *this = nullopt;
      this->construct(il, std::forward<Args>(args)...);
      return value();
    }

    /// Swaps this optional with the other.
    ///
    /// If neither optionals have a value, nothing happens.
    /// If both have a value, the values are swapped.
    /// If one has a value, it is moved to the other and the movee is left
    /// valueless.
    void swap(optional &rhs) noexcept(
      std::is_nothrow_move_constructible<T>::value && detail::is_nothrow_swappable<T>::value)
    {
      using std::swap;
      if (has_value())
        {
          if (rhs.has_value())
            {
              swap(**this, *rhs);
            }
          else
            {
              new (std::addressof(rhs.m_value)) T(std::move(this->m_value));
              this->m_value.T::~T();
            }
        }
      else if (rhs.has_value())
        {
          new (std::addressof(this->m_value)) T(std::move(rhs.m_value));
          rhs.m_value.T::~T();
        }
      swap(this->m_has_value, rhs.m_has_value);
    }

    /// Returns a pointer to the stored value
    constexpr const T *operator->() const { return std::addressof(this->m_value); }

    TL_OPTIONAL_11_CONSTEXPR T *operator->() { return std::addressof(this->m_value); }

    /// Returns the stored value
    TL_OPTIONAL_11_CONSTEXPR T &operator*() & { return this->m_value; }

    constexpr const T &operator*() const & { return this->m_value; }

    TL_OPTIONAL_11_CONSTEXPR T &&operator*() && { return std::move(this->m_value); }

#ifndef TL_OPTIONAL_NO_CONSTRR
    constexpr const T &&operator*() const && { return std::move(this->m_value); }
#endif

    /// Returns whether or not the optional has a value
    constexpr bool has_value() const noexcept { return this->m_has_value; }

    constexpr explicit operator bool() const noexcept { return this->m_has_value; }

    /// Returns the contained value if there is one, otherwise throws
    /// bad_optional_access
    TL_OPTIONAL_11_CONSTEXPR T &value() &
    {
      if (has_value())
        return this->m_value;
      throw bad_optional_access();
    }

    TL_OPTIONAL_11_CONSTEXPR const T &value() const &
    {
      if (has_value())
        return this->m_value;
      throw bad_optional_access();
    }

    TL_OPTIONAL_11_CONSTEXPR T &&value() &&
    {
      if (has_value())
        return std::move(this->m_value);
      throw bad_optional_access();
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    TL_OPTIONAL_11_CONSTEXPR const T &&value() const &&
    {
      if (has_value())
        return std::move(this->m_value);
      throw bad_optional_access();
    }
#endif

    /// Returns the stored value if there is one, otherwise returns `u`
    template <class U> constexpr T value_or(U &&u) const &
    {
      static_assert(
        std::is_copy_constructible<T>::value && std::is_convertible<U &&, T>::value,
        "T must be copy constructible and convertible from U");
      return has_value() ? **this : static_cast<T>(std::forward<U>(u));
    }

    template <class U> TL_OPTIONAL_11_CONSTEXPR T value_or(U &&u) &&
    {
      static_assert(
        std::is_move_constructible<T>::value && std::is_convertible<U &&, T>::value,
        "T must be move constructible and convertible from U");
      return has_value() ? std::move(**this) : static_cast<T>(std::forward<U>(u));
    }

    /// Destroys the stored value if one exists, making the optional empty
    void reset() noexcept
    {
      if (has_value())
        {
          this->m_value.~T();
          this->m_has_value = false;
        }
    }
  }; // namespace tl

  /// Compares two optional objects
  template <class T, class U> inline constexpr bool operator==(const optional<T> &lhs, const optional<U> &rhs)
  {
    return lhs.has_value() == rhs.has_value() && (!lhs.has_value() || *lhs == *rhs);
  }

  template <class T, class U> inline constexpr bool operator!=(const optional<T> &lhs, const optional<U> &rhs)
  {
    return lhs.has_value() != rhs.has_value() || (lhs.has_value() && *lhs != *rhs);
  }

  template <class T, class U> inline constexpr bool operator<(const optional<T> &lhs, const optional<U> &rhs)
  {
    return rhs.has_value() && (!lhs.has_value() || *lhs < *rhs);
  }

  template <class T, class U> inline constexpr bool operator>(const optional<T> &lhs, const optional<U> &rhs)
  {
    return lhs.has_value() && (!rhs.has_value() || *lhs > *rhs);
  }

  template <class T, class U> inline constexpr bool operator<=(const optional<T> &lhs, const optional<U> &rhs)
  {
    return !lhs.has_value() || (rhs.has_value() && *lhs <= *rhs);
  }

  template <class T, class U> inline constexpr bool operator>=(const optional<T> &lhs, const optional<U> &rhs)
  {
    return !rhs.has_value() || (lhs.has_value() && *lhs >= *rhs);
  }

  /// Compares an optional to a `nullopt`
  template <class T> inline constexpr bool operator==(const optional<T> &lhs, nullopt_t) noexcept
  {
    return !lhs.has_value();
  }

  template <class T> inline constexpr bool operator==(nullopt_t, const optional<T> &rhs) noexcept
  {
    return !rhs.has_value();
  }

  template <class T> inline constexpr bool operator!=(const optional<T> &lhs, nullopt_t) noexcept
  {
    return lhs.has_value();
  }

  template <class T> inline constexpr bool operator!=(nullopt_t, const optional<T> &rhs) noexcept
  {
    return rhs.has_value();
  }

  template <class T> inline constexpr bool operator<(const optional<T> &, nullopt_t) noexcept { return false; }

  template <class T> inline constexpr bool operator<(nullopt_t, const optional<T> &rhs) noexcept
  {
    return rhs.has_value();
  }

  template <class T> inline constexpr bool operator<=(const optional<T> &lhs, nullopt_t) noexcept
  {
    return !lhs.has_value();
  }

  template <class T> inline constexpr bool operator<=(nullopt_t, const optional<T> &) noexcept { return true; }

  template <class T> inline constexpr bool operator>(const optional<T> &lhs, nullopt_t) noexcept
  {
    return lhs.has_value();
  }

  template <class T> inline constexpr bool operator>(nullopt_t, const optional<T> &) noexcept { return false; }

  template <class T> inline constexpr bool operator>=(const optional<T> &, nullopt_t) noexcept { return true; }

  template <class T> inline constexpr bool operator>=(nullopt_t, const optional<T> &rhs) noexcept
  {
    return !rhs.has_value();
  }

  /// Compares the optional with a value.
  template <class T, class U> inline constexpr bool operator==(const optional<T> &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs == rhs : false;
  }

  template <class T, class U> inline constexpr bool operator==(const U &lhs, const optional<T> &rhs)
  {
    return rhs.has_value() ? lhs == *rhs : false;
  }

  template <class T, class U> inline constexpr bool operator!=(const optional<T> &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs != rhs : true;
  }

  template <class T, class U> inline constexpr bool operator!=(const U &lhs, const optional<T> &rhs)
  {
    return rhs.has_value() ? lhs != *rhs : true;
  }

  template <class T, class U> inline constexpr bool operator<(const optional<T> &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs < rhs : true;
  }

  template <class T, class U> inline constexpr bool operator<(const U &lhs, const optional<T> &rhs)
  {
    return rhs.has_value() ? lhs < *rhs : false;
  }

  template <class T, class U> inline constexpr bool operator<=(const optional<T> &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs <= rhs : true;
  }

  template <class T, class U> inline constexpr bool operator<=(const U &lhs, const optional<T> &rhs)
  {
    return rhs.has_value() ? lhs <= *rhs : false;
  }

  template <class T, class U> inline constexpr bool operator>(const optional<T> &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs > rhs : false;
  }

  template <class T, class U> inline constexpr bool operator>(const U &lhs, const optional<T> &rhs)
  {
    return rhs.has_value() ? lhs > *rhs : true;
  }

  template <class T, class U> inline constexpr bool operator>=(const optional<T> &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs >= rhs : false;
  }

  template <class T, class U> inline constexpr bool operator>=(const U &lhs, const optional<T> &rhs)
  {
    return rhs.has_value() ? lhs >= *rhs : true;
  }

  template <
    class T,
    detail::enable_if_t<std::is_move_constructible<T>::value> * = nullptr,
    detail::enable_if_t<detail::is_swappable<T>::value>       * = nullptr>
  void swap(optional<T> &lhs, optional<T> &rhs) noexcept(noexcept(lhs.swap(rhs)))
  {
    return lhs.swap(rhs);
  }

  namespace detail
  {
    struct i_am_secret
    {};
  } // namespace detail

  template <
    class T = detail::i_am_secret,
    class U,
    class Ret = detail::conditional_t<std::is_same<T, detail::i_am_secret>::value, detail::decay_t<U>, T>>
  inline constexpr optional<Ret> make_optional(U &&v)
  {
    return optional<Ret>(std::forward<U>(v));
  }

  template <class T, class... Args> inline constexpr optional<T> make_optional(Args &&...args)
  {
    return optional<T>(in_place, std::forward<Args>(args)...);
  }

  template <class T, class U, class... Args>
  inline constexpr optional<T> make_optional(std::initializer_list<U> il, Args &&...args)
  {
    return optional<T>(in_place, il, std::forward<Args>(args)...);
  }

#if __cplusplus >= 201703L
  template <class T> optional(T) -> optional<T>;
#endif

  /// \exclude
  namespace detail
  {
#ifdef TL_OPTIONAL_CXX14
    template <
      class Opt,
      class F,
      class Ret = decltype(detail::invoke(std::declval<F>(), *std::declval<Opt>())),
      detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
    constexpr auto optional_map_impl(Opt &&opt, F &&f)
    {
      return opt.has_value() ? detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt)) : optional<Ret>(nullopt);
    }

    template <
      class Opt,
      class F,
      class Ret = decltype(detail::invoke(std::declval<F>(), *std::declval<Opt>())),
      detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
    auto optional_map_impl(Opt &&opt, F &&f)
    {
      if (opt.has_value())
        {
          detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt));
          return make_optional(monostate{});
        }

      return optional<monostate>(nullopt);
    }
#else
    template <
      class Opt,
      class F,
      class Ret = decltype(detail::invoke(std::declval<F>(), *std::declval<Opt>())),
      detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>

    constexpr auto optional_map_impl(Opt &&opt, F &&f) -> optional<Ret>
    {
      return opt.has_value() ? detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt)) : optional<Ret>(nullopt);
    }

    template <
      class Opt,
      class F,
      class Ret = decltype(detail::invoke(std::declval<F>(), *std::declval<Opt>())),
      detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>

    auto optional_map_impl(Opt &&opt, F &&f) -> optional<monostate>
    {
      if (opt.has_value())
        {
          detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt));
          return monostate{};
        }

      return nullopt;
    }
#endif
  } // namespace detail

  /// Specialization for when `T` is a reference. `optional<T&>` acts similarly
  /// to a `T*`, but provides more operations and shows intent more clearly.
  template <class T> class optional<T &>
  {
  public:
// The different versions for C++14 and 11 are needed because deduced return
// types are not SFINAE-safe. This provides better support for things like
// generic lambdas. C.f.
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0826r0.html
#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54)                           \
  && !defined(TL_OPTIONAL_GCC55)

    /// Carries out some operation which returns an optional on the stored
    /// object if there is one.
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F &&f) &
    {
      using result = detail::invoke_result_t<F, T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }

    template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F &&f) &&
    {
      using result = detail::invoke_result_t<F, T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }

    template <class F> constexpr auto and_then(F &&f) const &
    {
      using result = detail::invoke_result_t<F, const T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F> constexpr auto and_then(F &&f) const &&
    {
      using result = detail::invoke_result_t<F, const T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
#endif
#else
    /// Carries out some operation which returns an optional on the stored
    /// object if there is one.
    template <class F> TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T &> and_then(F &&f) &
    {
      using result = detail::invoke_result_t<F, T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }

    template <class F> TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T &> and_then(F &&f) &&
    {
      using result = detail::invoke_result_t<F, T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }

    template <class F> constexpr detail::invoke_result_t<F, const T &> and_then(F &&f) const &
    {
      using result = detail::invoke_result_t<F, const T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F> constexpr detail::invoke_result_t<F, const T &> and_then(F &&f) const &&
    {
      using result = detail::invoke_result_t<F, const T &>;
      static_assert(detail::is_optional<result>::value, "F must return an optional");

      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : result(nullopt);
    }
#endif
#endif

#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54)                           \
  && !defined(TL_OPTIONAL_GCC55)
    /// Carries out some operation on the stored object if there is one.
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F &&f) &
    {
      return detail::optional_map_impl(*this, std::forward<F>(f));
    }

    template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F &&f) &&
    {
      return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }

    template <class F> constexpr auto map(F &&f) const &
    {
      return detail::optional_map_impl(*this, std::forward<F>(f));
    }

    template <class F> constexpr auto map(F &&f) const &&
    {
      return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#else
    /// Carries out some operation on the stored object if there is one.
    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional &>(), std::declval<F &&>()))
      map(F &&f) &
    {
      return detail::optional_map_impl(*this, std::forward<F>(f));
    }

    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional &&>(), std::declval<F &&>()))
      map(F &&f) &&
    {
      return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }

    template <class F>
    constexpr decltype(detail::optional_map_impl(std::declval<const optional &>(), std::declval<F &&>()))
      map(F &&f) const &
    {
      return detail::optional_map_impl(*this, std::forward<F>(f));
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F>
    constexpr decltype(detail::optional_map_impl(std::declval<const optional &&>(), std::declval<F &&>()))
      map(F &&f) const &&
    {
      return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#endif
#endif

#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54)                           \
  && !defined(TL_OPTIONAL_GCC55)
    /// Carries out some operation on the stored object if there is one.
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F &&f) &
    {
      return detail::optional_map_impl(*this, std::forward<F>(f));
    }

    template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F &&f) &&
    {
      return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }

    template <class F> constexpr auto transform(F &&f) const &
    {
      return detail::optional_map_impl(*this, std::forward<F>(f));
    }

    template <class F> constexpr auto transform(F &&f) const &&
    {
      return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#else
    /// Carries out some operation on the stored object if there is one.
    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional &>(), std::declval<F &&>()))
      transform(F &&f) &
    {
      return detail::optional_map_impl(*this, std::forward<F>(f));
    }

    /// \group map
    /// \synopsis template <class F> auto transform(F &&f) &&;
    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional &&>(), std::declval<F &&>()))
      transform(F &&f) &&
    {
      return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }

    template <class F>
    constexpr decltype(detail::optional_map_impl(std::declval<const optional &>(), std::declval<F &&>()))
      transform(F &&f) const &
    {
      return detail::optional_map_impl(*this, std::forward<F>(f));
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F>
    constexpr decltype(detail::optional_map_impl(std::declval<const optional &&>(), std::declval<F &&>()))
      transform(F &&f) const &&
    {
      return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#endif
#endif

    /// Calls `f` if the optional is empty
    template <class F, detail::enable_if_ret_void<F> * = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) &
    {
      if (has_value())
        return *this;

      std::forward<F>(f)();
      return nullopt;
    }

    template <class F, detail::disable_if_ret_void<F> * = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) &
    {
      return has_value() ? *this : std::forward<F>(f)();
    }

    template <class F, detail::enable_if_ret_void<F> * = nullptr> optional<T> or_else(F &&f) &&
    {
      if (has_value())
        return std::move(*this);

      std::forward<F>(f)();
      return nullopt;
    }

    template <class F, detail::disable_if_ret_void<F> * = nullptr>
    optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) &&
    {
      return has_value() ? std::move(*this) : std::forward<F>(f)();
    }

    template <class F, detail::enable_if_ret_void<F> * = nullptr> optional<T> or_else(F &&f) const &
    {
      if (has_value())
        return *this;

      std::forward<F>(f)();
      return nullopt;
    }

    template <class F, detail::disable_if_ret_void<F> * = nullptr>
    optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) const &
    {
      return has_value() ? *this : std::forward<F>(f)();
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, detail::enable_if_ret_void<F> * = nullptr> optional<T> or_else(F &&f) const &&
    {
      if (has_value())
        return std::move(*this);

      std::forward<F>(f)();
      return nullopt;
    }

    template <class F, detail::disable_if_ret_void<F> * = nullptr> optional<T> or_else(F &&f) const &&
    {
      return has_value() ? std::move(*this) : std::forward<F>(f)();
    }
#endif

    /// Maps the stored value with `f` if there is one, otherwise returns `u`
    template <class F, class U> U map_or(F &&f, U &&u) &
    {
      return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u);
    }

    template <class F, class U> U map_or(F &&f, U &&u) &&
    {
      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u);
    }

    template <class F, class U> U map_or(F &&f, U &&u) const &
    {
      return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u);
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, class U> U map_or(F &&f, U &&u) const &&
    {
      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u);
    }
#endif

    /// Maps the stored value with `f` if there is one, otherwise calls
    /// `u` and returns the result.
    template <class F, class U> detail::invoke_result_t<U> map_or_else(F &&f, U &&u) &
    {
      return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u)();
    }

    template <class F, class U> detail::invoke_result_t<U> map_or_else(F &&f, U &&u) &&
    {
      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u)();
    }

    template <class F, class U> detail::invoke_result_t<U> map_or_else(F &&f, U &&u) const &
    {
      return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u)();
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, class U> detail::invoke_result_t<U> map_or_else(F &&f, U &&u) const &&
    {
      return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u)();
    }
#endif

    /// Returns `u` if `*this` has a value, otherwise an empty optional.
    template <class U> constexpr optional<typename std::decay<U>::type> conjunction(U &&u) const
    {
      using result = optional<detail::decay_t<U>>;
      return has_value() ? result{u} : result{nullopt};
    }

    /// Returns `rhs` if `*this` is empty, otherwise the current value.
    TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional &rhs) & { return has_value() ? *this : rhs; }

    constexpr optional disjunction(const optional &rhs) const & { return has_value() ? *this : rhs; }

    TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional &rhs) &&
    {
      return has_value() ? std::move(*this) : rhs;
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    constexpr optional disjunction(const optional &rhs) const && { return has_value() ? std::move(*this) : rhs; }
#endif

    TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional &&rhs) & { return has_value() ? *this : std::move(rhs); }

    constexpr optional disjunction(optional &&rhs) const & { return has_value() ? *this : std::move(rhs); }

    TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional &&rhs) &&
    {
      return has_value() ? std::move(*this) : std::move(rhs);
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    constexpr optional disjunction(optional &&rhs) const && { return has_value() ? std::move(*this) : std::move(rhs); }
#endif

    /// Takes the value out of the optional, leaving it empty
    optional take()
    {
      optional ret = std::move(*this);
      reset();
      return ret;
    }

    using value_type = T &;

    /// Constructs an optional that does not contain a value.
    constexpr optional() noexcept : m_value(nullptr) {}

    constexpr optional(nullopt_t) noexcept : m_value(nullptr) {}

    /// Copy constructor
    ///
    /// If `rhs` contains a value, the stored value is direct-initialized with
    /// it. Otherwise, the constructed optional is empty.
    TL_OPTIONAL_11_CONSTEXPR optional(const optional &rhs) noexcept = default;

    /// Move constructor
    ///
    /// If `rhs` contains a value, the stored value is direct-initialized with
    /// it. Otherwise, the constructed optional is empty.
    TL_OPTIONAL_11_CONSTEXPR optional(optional &&rhs) = default;

    /// Constructs the stored value with `u`.
    template <class U = T, detail::enable_if_t<!detail::is_optional<detail::decay_t<U>>::value> * = nullptr>
    constexpr optional(U &&u) noexcept : m_value(std::addressof(u))
    {
      static_assert(std::is_lvalue_reference<U>::value, "U must be an lvalue");
    }

    template <class U> constexpr explicit optional(const optional<U> &rhs) noexcept : optional(*rhs) {}

    /// No-op
    ~optional() = default;

    /// Assignment to empty.
    ///
    /// Destroys the current value if there is one.
    optional &operator=(nullopt_t) noexcept
    {
      m_value = nullptr;
      return *this;
    }

    /// Copy assignment.
    ///
    /// Rebinds this optional to the referee of `rhs` if there is one. Otherwise
    /// resets the stored value in `*this`.
    optional &operator=(const optional &rhs) = default;

    /// Rebinds this optional to `u`.
    template <class U = T, detail::enable_if_t<!detail::is_optional<detail::decay_t<U>>::value> * = nullptr>
    optional &operator=(U &&u)
    {
      static_assert(std::is_lvalue_reference<U>::value, "U must be an lvalue");
      m_value = std::addressof(u);
      return *this;
    }

    /// Converting copy assignment operator.
    ///
    /// Rebinds this optional to the referee of `rhs` if there is one. Otherwise
    /// resets the stored value in `*this`.
    template <class U> optional &operator=(const optional<U> &rhs) noexcept
    {
      m_value = std::addressof(rhs.value());
      return *this;
    }

    /// Rebinds this optional to `u`.
    template <class U = T, detail::enable_if_t<!detail::is_optional<detail::decay_t<U>>::value> * = nullptr>
    optional &emplace(U &&u) noexcept
    {
      return *this = std::forward<U>(u);
    }

    void swap(optional &rhs) noexcept { std::swap(m_value, rhs.m_value); }

    /// Returns a pointer to the stored value
    constexpr const T *operator->() const noexcept { return m_value; }

    TL_OPTIONAL_11_CONSTEXPR T *operator->() noexcept { return m_value; }

    /// Returns the stored value
    TL_OPTIONAL_11_CONSTEXPR T &operator*() noexcept { return *m_value; }

    constexpr const T &operator*() const noexcept { return *m_value; }

    constexpr bool has_value() const noexcept { return m_value != nullptr; }

    constexpr explicit operator bool() const noexcept { return m_value != nullptr; }

    /// Returns the contained value if there is one, otherwise throws
    /// bad_optional_access
    TL_OPTIONAL_11_CONSTEXPR T &value()
    {
      if (has_value())
        return *m_value;
      throw bad_optional_access();
    }

    TL_OPTIONAL_11_CONSTEXPR const T &value() const
    {
      if (has_value())
        return *m_value;
      throw bad_optional_access();
    }

    /// Returns the stored value if there is one, otherwise returns `u`
    template <class U> constexpr T value_or(U &&u) const & noexcept
    {
      static_assert(
        std::is_copy_constructible<T>::value && std::is_convertible<U &&, T>::value,
        "T must be copy constructible and convertible from U");
      return has_value() ? **this : static_cast<T>(std::forward<U>(u));
    }

    /// \group value_or
    template <class U> TL_OPTIONAL_11_CONSTEXPR T value_or(U &&u) && noexcept
    {
      static_assert(
        std::is_move_constructible<T>::value && std::is_convertible<U &&, T>::value,
        "T must be move constructible and convertible from U");
      return has_value() ? **this : static_cast<T>(std::forward<U>(u));
    }

    /// Destroys the stored value if one exists, making the optional empty
    void reset() noexcept { m_value = nullptr; }

  private:
    T *m_value;
  }; // namespace tl

} // namespace tl

namespace std
{
  // TODO SFINAE
  template <class T> struct hash<tl::optional<T>>
  {
    ::std::size_t operator()(const tl::optional<T> &o) const
    {
      if (!o.has_value())
        return 0;

      return std::hash<tl::detail::remove_const_t<T>>()(*o);
    }
  };
} // namespace std

#endif
